A high-viscosity glycoglucuronomannan from the gum exudate of Vochysia thyrsoidea:: Comparison with those of other Vochysia spp.

The polysaccharide (VTh) extracted with water from the gum exudate of the trunk Vochysia thyrsoidea gave a solution of high viscosity and was homogeneous, with M-w 92.5 x 10(3) and dnldc = 0.212. Partial hydrolysis gave a main chain of -> 4)-beta-D-GlcpA-(1 -> 2)-alpha-D-Manp-(1 -> repeating groups. Since VTh contained 24% GIcA, neither its monosaccharide ratio (GC-MS of alditol acetates) nor methylation results were accurate. It was therefore carbodiimide-reduced to CR3VTh, which contained Ara, Xyl, Man, Gal, and Glc in a molar ratio of 28:4:29:19:25: methylation analysis showed mainly nonreducing end-units of Araf (25%), Arap (8%), and Galp (8%) and side-chain units of 3-O- (6%) and 3,4-di-O-subst. Galp (12%). Main-chain units of Manp were 2,3-di-O-substituted (17%), showing that they were substituted at O-3 by side chains, but there was less side-chain substitution of Glcp units, which were mainly 4-O-substituted (12%). Most Araf units in VTh and CR3VTh were single nonreducing end-units, and were present in groups of alpha-L-Araf-(1 -> 3)-D-Manp and alpha-L-Araf-(1 -> 3)-[alpha-L-Araf-(1 -> 4)]-beta-D-Galp-(1 -> 3)-alpha-D-Manp. Other degradation products derived from VTh agreed with this structure. The degree of side-chain substitution of the same main chain was greatest with the gum exudate polysaccharide of V tucanorum, and progressively less with those of V. thyrsoidea, and V. lehmannii. High-viscosity aqueous solutions were formed by the gum of V. thyrsoidea, in contrast with those of V. lehmannii and V. tucanorum. (C) 2007 Elsevier Ltd. All rights reserved.